IBL with cycles: can I use a different background image from the one that affects the model?
The Cycles render engine uses a ray-traced system, in order to render, so there are multiple rays casted while rendering, and you can use the Lightpath node, in order to determine the type of ray; Take a look at this node setup:



You will get this effect:



Note that I added a RGB Curves node in order to reduce the brightness and contrast of a instance of the image, which is only visible to the camera; The white circle and oval in the image represent the same area, yet look different. Maybe I exaggerated the effect a little bit...

Also, you can do logical operations such as OR or AND in order to use multiple ray types; If you want more details on this, ask it as a new question.